I'd have a quick check about the GSM board's requirements... I thought most of them had a 3.3v regulator on-board so you may find you can run them directly off 3.7v, which makes you life a lot easier.
If so that makes your life a lot easier. Just get either some phone batteries and connect them in parallel (Adafruit sell a charger board but it only handles a single cell's worth of voltage)
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
I'd have a quick check about the GSM board's requirements... I thought most of them had a 3.3v regulator on-board so you may find you can run them directly off 3.7v, which makes you life a lot easier.
If so that makes your life a lot easier. Just get either some phone batteries and connect them in parallel (Adafruit sell a charger board but it only handles a single cell's worth of voltage)